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RE: Modulus of Subgrade reaction at mat foundation

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The key to understanding this reduction is considering
the volume of soil influenced by that square foot of
bearing.  Look at the stress contours in the soil
beneath the mat.  The one foot square isolated footing
in order to deflect must also distort the soil
adjacent in addition to that directly below the
footing.  When you look at a large mat the impact of
the adjacent soil is much smaller percentage wise.

The subgrade modulus assumes that you have isolated
springs below your footing while in fact you have a 3D
continuum.

In addition the subgrade modulus is non-linear and
varies with the intensity of the load.  This typically
results in an iterative process whereby you will
provide the Geotech with soil pressures and he will
provide you with a revised subgrade modulus for
portions of your mat. 

Mark Gilligan

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